In many clinical evaluations it is necessary to obtain a sample of a patient's blood for analysis. Many of the specific analytical procedures require only the serum fraction of the patient's blood. Previously, practitioners generally used a hypodermic needle and syringe to make a penetration into the patient's vessel to draw a desired sample. The sample was then generally transferred to a tube, allowed to clot, then subjected to centrifugation to separate the serum fraction from the cellular fraction. With the advent of evacuated glass blood collection tubes, practitioners were able to use the tube in a needle holder with a double ended needle to collect the sample and then centrifuge the tube to separate the serum fraction. Since glass presents a hydrophilic surface that promotes clotting to the blood sample, the blood sample clotting process is readily initiated. When the tube containing the clotted blood is centrifuged, the cellular components do not tend to adhere to the glass, thus providing a substantially cell-free serum fraction for the desired analysis.
With the wide-spread recognition among medical and laboratory practitioners of the dangers of blood borne pathogens, manufacturers have begun to supply blood collection tubes formed from polymeric materials that are substantially more resistant to shattering caused by inadvertent mishandling than are the conventional glass tubes. While these polymeric tubes substantially reduced the incidence of shattered glass tubes, the polymeric materials, commonly polyethylene, polycarbonate, polyethylene terephthalate and the like, used to replace glass, present a hydrophobic surface to the blood samples. In an untreated polymeric tube, clotting is often difficult to initiate, and after centrifugation, there often is red blood cell hang-up or red cell film on the surface of the tube which may result in serum samples being contaminated with cells. A generally accepted definition of red cell hang-up is clumps of red blood cells or a substantially opaque coating of red blood cells on the tube wall. Red cell film is generally defined as a substantially sheer translucent coating of red blood cells on the tube wall.
U.S. Pat. No. 5,246,666 discloses additives for blood collection tubes that initiate clotting and induce the higher density cellular material to agglomerate at the bottom of the tube when it is centrifuged. While the disclosed additives assist the agglomeration of the cellular material during centrifugation, the problem of the cell hang-up and film on the tube walls still exists when polymeric materials are used to form the tubes.
U.S. Pat. Nos. 3,852,194 and 4,101,422 disclose addition of thixotropic gel materials to tubes that facilitate the separation between heavier and lighter blood fractions during centrifugation. The manufacture of these thixotropic gels must be carefully controlled and tubes containing these gels may have a shorter shelf life than the non-gel containing tubes. Additionally, the thixotropic gels may develop agglomerations or globules with lower specific gravity that separate from the bulk gel during centrifugation and interfere with the separation. Also, certain thixotropic gel materials may selectively absorb analytes from the blood samples thus confounding subsequent analytical procedures performed on the samples.
If a polymeric tube were available that additionally had surface properties to substantially eliminate red cell hang-up and film, that was substantially inert and unextractable from the tube inner surface and did not substantially reduce the efficiency of tube manufacture, the art of blood collection tubes would be advanced. Such a tube and a method for its manufacture are disclosed hereinbelow.